Speech inverter



Nov. 21, 1950 L KRS SPEECH INVERTER Filed oct. 15, 194e Patented Nov. 21, 1950 SPEECH INVERTER Ladislao K-rs, Buenos Aires, Argentina, assigner to Radio Corporation of America, a corporation of Delaware Appl-icationoctoher 15, 1946, Serial No. v703,435

connection, a speech inverter arranged .in accordance with my invention, while Fig. 2 illustrates .a low pass filter circuit vsuitable for use in my speech inverter.

There are in use different kinds of speech .inverters in the telecommunication service.. The .common practice is t let the intelligence :dow through a low-pass vfilter which .prevents those higher frequencies which are .not important for the intelligibility of the speech from .passing into the following circuits. This limit will often be chosen at 2700 C. P. S. so that the rst filter will allow .to pass fs 2700 C. P. S.

This frequency .spectrum will be introduced im to .a modulator together with a carrier frequency, fc, which will be modulated with these js frequencies. The usual fc=3000 C. P. S. The modulator is generally a balanced one and produces different frequency groups. The main groups are the fc4-fs and fL--fs frequencies With different parasitic frequency groups. The output intelligence from the modulator iiows through a second .low pass lter. The cut-,off frequency of the second lowpass ,filter could be the same as the cut-01T frequency of the first lter, for example, 2700 C. P.. S. so that the second low-pass iilter'allows the fc--fs frequency .spectrum and some of "the vparasitic groups to pass. Using f=3000 C. P. S. and the cnt-off frequency 27o@ vC. P. S. for the filters,

(Cl. 17g-1.5)

12 .mission service or will .be sentlover the 'telephonie line for telephone service.

In the receiving end the fc-fs frequencies 'will be treated .in exactly the same manner. as .fthe original input intelligence in .the transmitting equipment. The inverted intelligence will flow through the receiving channel of Vthe yspeech inverter and ina similar manner the .fc-Oc-js) :L frequency .group willv be selected again, which is the .sending end 'input frequency. The ,fi-fi irequenoy group emittedfrom the sending end cannot be understood with a common ,receiver so that the system assures privacy for those who have at their. disposal the described speech inverter apparatus.

.In the practical designs, `often the well known system of balanced modulator will .be used to mix the fs intelligence with .the fc frequency. The ideal case that only the ,ft-fs frequency y.group .and nothing else should be transmitted is not generally fulfilled. In the samer-manner in the receiving end the .finally produced ,frequency .group contains other frequencies .than the originally transmitted fs;

There exist .different vmodulator systems. VEach system produces besides the ,fcifs frequency group, other frequencies. Four different combinations of parasitic frequency groups, shown in Table I, -are produced according to the different circuits of the modulators. In the literature there are data about modulator systems, for example, Applied Electronics, editor John Wiley 8: Sons, edition 1'1/43, pages 688 to 690.

TABLE I Parasitic Fre- Case quency Groups From the fcifs frequency group, produced fby the modulator, the above mentioned .low pass iilterfgenerally with the fc=innite attenuation frequency, cancels out the fc4-f3 group, .but 'cannot cancel out the parasitic frequency groups of Table I, insofar as these parasitic frequencies belowlthe ,fc frequency.

It is a further object of my invention to eli-minate or atleast reduce inthe system, parasitic frequencies of the groups listed in Table I, "to thereby insure fairV intelligibility `of the inverted signal.

It can be demonstrated that with a good choice of the relation of the level of fs and fc, the parasitic frequency groups can be reduced to a low limit that will assure a fair intelligibility of the received double inverted intelligence.

In Fig. 1, I have illustrated a signalling system of the general type described above arranged to include novel features kof my invention, means to reduce said parasitic frequencies. The intelligence introduced in the transformer l' come from a subscriber station SS by way of a hybrid coupling HC. The transformer may supply speech currents from its secondary winding through an attenuating network comprising resistors 2, 3 and e to a iirst filter supplying the primary winding of the transformer 6 having two primaries. The other primary winding of the transformer 6 may be coupled by an amplifier tube 32 to an oscillator O. This coupling includes the condenser 58 and grid bias resistor 33, between oscillator O and the input of tube 32 and the primary winding of transformer 5 coupled to the anode of tube 32. This winding is also in v the D. C. path to the plate source.

The gain of the amplifier tube 32 is controlled kin accordance with my invention, as will appear in. detail hereinafter, in a direction to expand the amplitude of the oscillations fed to the transformer 6 as the signal level in transformer l rises.

"Tube 9 comprises two diodes in a balanced modulator circuit and also a triode in an amplifier circuit. The diodes are in a balanced converter circuit including load resistor Il and shunt condenser I0. The grid of the triode is coupled by condenser 8 to load l l The resistor l2 is for bias purposes. The operation of the balanced modulator and tube 9 will be understood by those versed in the art and will be described herein- 'after.` The output of the balanced modulator, amplified in the tube 9, is supplied to the transformer i4 and thence to a second filter SF and kto a double tube I8 wherein it is amplified and supplied to the output transformer 28. The tube '|`8 is a double triode in a resistance-capacitive coupled amplifier. Resistor I1 is the grid bias resistor. Resistors VI9 and 22 are cathode return 'resistors andcondensers 20 and 2l are for bypass purposes.

Resistor z-forrns the anode load for 'the first triode and capacitor 24 feeds the signals to the grid of the second triode. The output from *the transformer 28 may be supplied to a transvmitter and/or a line as desired. In my use of the speech inverter, the line output supplies a radio transmitter.

The receiver channel is similar to a large eX- tent, to the transmitter channel. Signals might 4`be derived from a radio receiver' RR and supplied as input to a transformer I and from the transformer to an attenuating network and so forth.

In the receiver channel, I have used numerals corresponding to the numerals used in the transmitter channel except that the numerals in the receiver channel have been primed. rlhe receiver output from transformer 28 might be supplied 'by `way of the hybrid coupling unit I-IC to the l'subscriber station.

f* In accordance with my invention, oscillations rnight lator O and amplified by tube 32.

more, in accordance with my invention, the signals from the transformer i are supplied by resistance 3i and coupling condenser 45 to the control grid of a double triode tube 41, the rst section of which is an amplier. The envelope of tube M also includes a second triode in a diode rectifier circuit. The amplified signals are supplied by condenser 48 to the anode and control grid of the rectifier section of the tube which are tied together and the rectifying action sets up in resistor 53 and potentiometer resistor 52, a potential which waxes and wanes as the level of the rectified signal changes. This potential is supplied by resistor 54, filter network comprising resistor 55 and capacitors 58 and 57 to a control grid SC in tube 32 to control the gain of this tube. Note that as the signal level at the transformer i rises, the potential across resistor 52 towards the diode cathode end becomes less negative as does the potential supplied to the control grid SC of tube 32 so that the gain of tube 32 goes up when the signal level rises and the amplitude of thev oscillations in the primary winding of transformer 6 rises in a corresponding manner. Tubes 32 and 4i" operate in a corresponding manner to expand the amplitude of the oscillations from O supplied by tube 32 to the primary winding of 6' when. the signal level in the secondary winding of transformer l rises.

In Fig. 1, the intelligence is introduced in the input transformer l and flows through the nrst filter. In the transformer 6 is added the fc=3000 C. P. S. fc could be chosen for another frequency, the 3000 C. P. S. being given only as an example. The invention is not limited to the choice of any value of frequencies.

The fc frequency will be produced by the oscil- The plate transformer of 32 is the above mentioned transformer 6. Thetwo diode rectiflers of 9, with the load resistor li and condenser i0, will modulate the intelligence with the carrier frequency according to the well known principle of the balanced modulator and the triode part of the same tube 9 will amplify the product. A similar circuit is used in a speech inverter used by mv assignee, which is in turn similar to the speech inverter shown in Jacobi et al. application Serial #599,830, iiled June 16, 1945. In the plate circuit of the triode part of 9 and consequently in the transformer il! the fai-fs frequency group and the 2h and 2f@ frequency groups will be present as in the case i of Table I.

The invention is not limited to the use of this form for mixing fs and fc. Fig. 1 shows only one practical realization of a speech inverter, and its modulator could be substituted in any other system. To avoid repeating descriptions, in this speciiication, this form of the modulator will be used, but any other modulator system and freexample the first maximum attenuation frequency of the iirst and second lters. The second lter SF will attenuate heavily the fc4-fs frequency groups and the 2f@ frequency and all those 2fs frequencies where but will let pass from the parasitic frequency groups of case 4, Table I, the 2fs fc frequencies.

andere .Imm-etica `tlfieiattenuation:coulcl'lzae:about 3.0 to 60 db. for those frequencies where the low pass lterettenuates, i; erover fer-.300.0 -.C. P.. S.

i'heisymbolof (Zf.) will'loe used for all ofthe 213. frequencies-.belowthe; fc limit. (2n) Vywill remain putputV together with the inverted in.- telligence ir--fs and :represents a distortion'. This @fel isfin one .octave relation with the incomingintelligence, i e., .itzis'not inverted. If the leveLoff-the (27%?) groupis high enough, the inputdirect :intelligence can be understood with acommon receiver. `The .doubling vof the input frequencies does. not impede the vintelligibility er the speeclny thisnevent then purpose of the speech inversion, which isl privacy, will not be fulfilled. Experience showsv that the. parasitic (2f-t) frequency groups'can be reduced toa vnegligible value :by using'about 201db. higher level for'iethanfor f-swin the modulator which is formed by the diodes of thev tube 9; in Fig. 1. Using a higher level difference than to 30 db. between -fs and fe, the (2h) frequency-group `will not :be reduced appreciably more, but the -faand 2f.: frequencies will `be increased in the input and output unnecessarily.

The fc and 2fe frequencies in the input and outputV can cause disturbance; The fc and the 2jr; frequenciesgcan be` :reduced toV inaudibilityr in the'output bythe secondlter andthe fc frequency can also berreduced to inaudibility in the input line by the rst filter, provided that .the level .of .the fc frequencyisselected in accordance with the above description. Let us suppose for a -practical'case that the .attenuation for'the first lter is 50 db. for f=3000 Cil?. S. and ifor the 2fc=6000 C. P. S. .and Athe .attenuation of the second lter is also 50 db., and that we use 20 db. higher fc level than the intelligence. The sup 'posed attenuationnof 5.0 db. is a very good -value whichwill be found in exceptionally well. made speech inverter equipment. The output and the input line will :have in this case 50 Clo-20 db.=30 db. level in the fc and 2fc if the intelligence stops or if its level is lowered.

We will investigate anumeric example:

The normal conversation speech, fs, contains level dilferences of 26 db.. between its loudest and lowest periods. For example, we suppose that ina commonspeech inverter system the level of the fe frequency'in the diodes of tube 9 will be selected 20 db. higher than the loudest js frequency, and no output amplifier is used.

The absolute peaklevel should be +l0db. (0 level=1 mvv: in 500 ohms) at the diode plates of 9. In this case, the'v-absolute level of )2:3000 C.'P; S.. will 'be -10 cib-.+20 db.=+30 db. at the same diode plates. In .the output and input lines, after the rstandsecond filters, the 60.00 and3000 C. P. VS. vabsolute level will be +30 db.-501db=-20 db'. In this calculation-we suppose that the level of 3000 and 6000 C. P. S. s the same, which will .be fulfilledgapprcximatey in a general case;

We will investigate now the situation with the minimum level ofspeech, which vis the absolute level of +10 db.-26 db.=-16 db., supposing the level differences in speech `amount to 26 db. We see that-'the noise level from the 3000and 6000.0. AP. S. below the minimum vspeech level Til is' only 4. db.; fi. e. -l6..db.-(-;20 db.-)=4 db. This 'means practically that the lowest'levels .of intelligence cannot 'be understooddue vto the high 6000 and-3000`C. P. S..noise.level. If the intelltgence stops, v.the line carries 20.db. 600001- 3000 C. P` iS. noise which might be very. inconvenient as'disturbance produced in parallel lines provided that zthe balaneeand insulation between V.them is imperfect asis very oftenthe case.

Ingood engineering practice, the maximum attenuation for speech inverterlters vis about-30 '50,560 db; The relative noise level in Jthe lines could .be-improved theoretically by using a higher attenuation for :the 3000'and` 6000 C. P. S; filters. The diiiculty in this case will be in the frequency response ofthe vintelligence with normal filter dimensions, by using common quality materials.- Q=20 is a fair value for one lter element. Using 3,7filter Isections in the yrst and second filter :as shown in Fig. 2, forl example, the frequency response of i4 db. can be assuredbetween 300 and 2700 C. P; S. for the inverted intelligence, that is, about the maximum diiference that can be'tol'erated. vUsing more lter sections ofthe same quality, the frequency response will be worse. Without increasing the size and the pro.- ducticn cost of the speech inverter, the common speechinverters have no possibility of producing a service with less noise. The `noise level canv alsor'iseeasily if the fc frequency, which will be produced generally by an oscillator, has any drift. Iffthe cz3000 C. P. S. is changed for example, to Z900-C. P. S. the attenuation of the filter normally db. could .becomel -15 db., so that the noise'V level of v the new -f=29001 and `2fc=5800 rises considerably.

Another incorrectV attempt to reducethis noise in the common speech inverter systems is4v the reduction ofthe level of the fc='3000 C. P. S. in order to lower the `noise level. It theflevel of fe were only 10 db. and-not 20 db. higher than th peak of the intelligence, the dierence between the lowest intelligence and the level of 6000 or 3000 C. P. S. would be 14 db. after the filter and not 4 db. as calculated before, andthe absolute level of fein the'line -30 db. and not -20 db. as-before. This value is not good enough from the noiseless transmission standpoint, but this arrangement is also bad from the secrecy'standpoint. The peaks of the speech, `having only 10 db. difference between fs and fc, would pass asr a double frequency and could be understood wit-h common receivers. Entire words could cross directly in this arrangement; Also in the receiver end when listening with` a speechinverter, these direct crossing words wouldV makeV serious distortion, being inverted at this point and the intelligibility decreasing ap#- preciably.

This-invention gives what I believeis a perfect solution to these problems; With it -the level of the fc will be controlled in accordance with the incoming intelligence level, so that a previoush; fixed difference will be assured between the fc and fs levels. This diiference could be, for eX-- ample, 20 db., therefore there will be at all times enough fc present in the modulator. The (2)2) group remains low atv all times and on the other hand the fc and 2f@ frequencies,` because of their lxed level'relation with the intelligence, cannot disturb the line. When the intelligence stops;` the fc level could be reduced to a very Vlow value, which' cannot bedetected in the line. In Fing. lV this change of fc level will 'be produced by the tube32. Ihavechosen in this special example for the tube 32 the RCA 6L'7 tube. The tube 32 works in the well known form of a volume expander. The grid of one section of the double triode 41, which is an RCA 6SN7, is coupled with the attenuator resistors 3|, 45 and condenser 45 to the input transformer I. The second section of the tube 41 works in this case as a diode rectier and produces with its lter system; resistors 52, 54, 55 and condensers 5|, 56 and 51; a direct current control voltage, the value of which depends on the input level. It will be noted that instead of one double triode in place of'41, any triode or pentode amplifier with any rectier could be used, and this invention is not limited whatsoever by the use of any type of tube. In special cases, the amplifier stage could be omitted, provided the input level is high enough to drive the rectifier.

The eiiect of fc=3000 C. P. S., in the input grid of tube 41 is reduced by the first filter and an attenuator formed by the resistors 2, 3 and 4.

The first grid of the tube 32 will be excited by the ,fc frequency oscillator O. The oscillator can be formed by one or more electronic tubes in the well known manner, or may be a rotating generator. The supercontrol grid SC of 32 is biased negatively to a value so that the output from 32 on the diode plates of tube 9 has a low value, for example, db. 'This is the standby position of the speech inverter. If input intelligence is present, the input grid of the tube 41 will be excited and the direct current voltage developed by the diode of 41 will neutralize more and more the negative bias of the supercontrol grid oi tube 32 and the level of fc increases over the diode plates of 9. By the correct choice of the voltages and the resistance values, it is possible to assure that the level of the fc will follow the input level with aprexed diierence and in this form the correct inversion of the speech for any input level will be assured. In a case where a very wide dynamic range, such as 30 db. level difference or more is applied to the input, the modulation process has its necessary level of fc and never more than necessary. Words pronounced with exceptional loudness cannot pass directly in this system because the necessary fc level will always be assured from the input level of the speech itself. Peaks outside of the mentioned 30 db. dynamic range are also not transmitted directly. The expander will be driven to a point in its range where it is no longer working in a linear form. This means that the distortion can increase for the peaks, but the intelligence does not cross the speech inverter directly. If the input intelligence is lower, the fc level becomes automatically lower also, so that the noise level of the 2f@ and ,fc is for any input level db.-20 db.=30 db. lower than the intelligence which is present in the line, supposing that the iilters have 50 db. attenuation for 2fc and fc and the level of the fc is 2O db. higher than the intelligence. This difference of 30 db. is a fair value and cannot be detected below the intelligence.

If the intelligence stops, the absolute level of fc in the modulator of 9 can be as low as -30 db. In this case -30 db.-50 db.=80 db. is the level of 2f@ and fc in the line, having at all times the attenuation of 50 db. from the lters. This attenuation until 80 db. in the line is an excellent result which could practically never be obtained with the common speech inverter systems. This 80 db. signal level will be attenuated more on the input side, having the attenuator 2, 3 and d in the path of the fc frequency which could produce in a practical case an additional attenuation o about 6 to 12 db. v

In the output line the absolute level of the 2fc together with the intelligence, will be amplified by the triode unit of the tube 9 and by the out-y put amplier I8. It should be mentioned that this invention is not limited to the use of a double diode and triode unit 9 and also not to the use of a double triode I8 and finally it has no importance for this invention if any amplifier tube is present or not after the modulator, although the ycircuit shown in Fig. lis a good practical example of a speech inverter designed for using the principles of this invention.

Au objection which may arise to this system could be that the time constant of the expander given by the resistances 52, 54, 55 and condensers 5|, 56 and 51 could disturb the exact form of the operation of this invention because of not having enough level of fc over the diode plates of 9, at the moment when the intelligence starts. On the other hand the fc does not disappear immediately once the speech has stopped. Experiments show that fortunately those theoretical objections do not make any audible effect, provided the time constant of the expander system isfairly short. A multi-stage resistor-condenser lter system like that shown in Fig. 1 gives'very good results, producing the necessary iilter eiect for the direct current voltage for the bias of the tubes 32 and relatively short time constant. In Table II are given several values for the important resistances and condensers of a practical construction like Fig. 1 of the present invention, for: the use of c=3000 C. P. S.

TABLE II Figure 1 Value Ohms Capacitors Value pf 1o, as 34, 41

cannot occur via the fc, oscillator because the ex-l.

pander tubes 32 and 32 are working like separators between them. The plate supply is gene. erally common for the two channels. The de` coupling elements; resistors in combination with condensers I5 and H5, l5 and l; 35 and 25, 3 and 29'; l2 and 39, 42 and 392:13 and 59, t3 and.

59 or neon voltage stabilizers may be used in the common way to prevent a cross-talk through the power supply.

It will be noted that no special explanation was given for such elements of Fig. l as are commonly used in ordinary amplifier technique. The resis- 9 tor I2 is the bias resistor for the tube 9 which could be an RCA 6SR'7, and the condenser I3 is its by-pass condenser. 8 is the grid coupling condenser and 'I the grid resistance for the triode part of `EI. The transformer I4 is the coupling between the plate of the triode unit of 9 and the second filter. The second filter is loaded by the resistor Il. I9 is the bias resistor for the rst triode of I3. IS Imay be an RCA GSN'Z tube. 22 is the bias resistor for the second triode of the same tube. and 2i are the by-pass condensers for the bias resistors. 24 is the coupling con- 'denser between the two triode units of I8, and 23 the grid resistor of the second triode, while 25 is the plate resistor of the rst triode of I8. 28 is the output transformer of the transmitter channel. The primary Winding of 28 is in the plate circuit of the second triode unit of IB. 58 is the coupling condenser of the first grid of 32; 33 is the grid resistance of the same. Resistors 35 and 50 are delivering the grid bias for the rst and third grid of 32. The voltage dividers 35 and 3l supply the screen grid voltage of the Ysaine tube. 38 .and 39 are the by-pass condensers for the screen and plate circuits of the same tube. 135 is the gridresistor for the triode part of tube 41, which tube may be an RCA 6SN7; 49 is the cathode bias resistor for the same unit and is its by-pass condenser. 4'8 is the coupling condenser between the triode and diode unit of 4l. The diode rectifier is made by short cii'cuiting the plate and grid of the second unit. The resistor 53 is parallel with the tube in accordance with the well known principle of the tube voltmeters. In

resistor 52 and condenser 5I the compensating i grid voltage will be developed for the third grid of 32, which grid voltage will be supplied by the iilter system, described previously. The receiver channel has the same elements as the transmitter channel. The corresponding parts in the receiver channel carry `an apostrophe.

It is clear that this system may be combined in diierent forms, using other circuits than shown in Fig. 1 with diierent values of the elements or by using a different kind of modulator system.

All the descriptions given in this paper for Fig. 1 and 2 and Table II represent only one practical design iroin the numerous other possibilities of this invention. f'

What is claimed is:

1. In a speech inverter having an input to which speech signals are applied: an attenuator coupled to said input for attenuating said signals; a iilter coupled to the output of said attenuator; a modulator coupled to the output of said lter receptive of carrier energy and of filtered attenuated speech signals; and means for maintaining a predetermined ratio between the level of the carrier energy supplied to said modulator and the level of the speech signals Y impressed on the modulator, said means including an amplier tube having a gain-controlling electrode therein and having an output electrode, a source of carrier energy, means for applying carrier energy from said source to'said tube for controllable-gain amplification of the lsame therein. means coupling said gain-controlling electrode to said input to make the gain of said LSI) tube responsive to applied speech signals, {said Y rectifying speech signals derived from said input, and means connecting said output electrode to said modulator to supply carrier energy thereto from said tube.

2. In a speech inverter with two channels each having an input to which speech signals are applied and each having output connections: a lter for each channel; means coupling each of said filters to a corresponding one of said inputs; a separate modulator coupled to the output of each filter receptive of carrier energy and of ltered speech signals in the corresponding channel; means for maintaining a predetermined ratio between the level of the carrier energy supplied to each modulator and the level of the speech signals impressed on the same modulator, said last-mentioned means including an ampliner tube for each channel, each tube having a gain-controlling electrode therein and having an output electrode, a common source of carrier energy for the two channels, means for applying carrier energy from said source to each of said tubes for controllable-gain ampliiication of the same therein, means connecting each of the output electrodes to a corresponding one of said modulators to supply energy thereto from the corresponding amplifier tube, and means coupling each gain-controlling electrode to the input of that channel corresponding to the modulator to which the corresponding amplifier tube output is connected, thereby to control the gain of the respective amplifier tube in accordance with the average magnitutde of speech signals fed to that modulator; a separate filter coupled to the output of each modulator; and means coupling the output of each of said last-mentioned filters to the output connections of the corresponding channel.

3. In a speech amplier system having an input to which speech signals are applied, an os cillatory energy generator of carrier frequency, a modulator coupled to said input, receptive of carrier energy and of speech signals, for modulating said carrier energy by said signals, an amplifier of controllable gain coupling said generator to said modulator to feed controllablyamplified carrier energy to said modulator, and means for controlling the gain of said ampliiier in accordance with the signal magnitude at said input, to make the level of the oscillatory energy of carrier-frequency fed to the modulator substantially twenty decibels higher than the level of the speech signals impressed on the modulator.

LADIsLAo Kns.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,563,326 Bown Dec. l, 1925 1,571,010 Kendall Jan. 26, 1926 1,806,666 Bown May 2-6, 1931 2,014,081 Csepely Sept. 10, 1935 2,029,389 Ringel Feb. 4, 1936 2,207,249" Goldsmith June 9, 1940 2,402,059 Crab June 11, 1946 

